Microsoft released several software patches on Wednesday for a bug that could let seemingly harmless image files open PCs to hackers.

Independent security researchers discovered the flaw and alerted the software company to its existence in March 2005. It affects most versions of Microsoft Windows and received the company's highest alert rating of "critical".

The bug allows modified image files to bypass the security of the Windows operating system, potentially providing remote access to an outsider. Images saved in the Windows Metafile and Enhanced Metafile forms – with the extensions .wmf and .emf – could be converted to attack PCs, experts say.